This invention relates to a liquid-crystal display device and a driving method thereof, and more particularly to a liquid-crystal display device using a liquid-crystal materiel with a hysteresis characteristic, such as cholesteric liquid crystal, and a driving method thereof.
There have been liquid-crystal display cells using liquid-crystal material (Polymer Stabilized Cholesteric Texture: PSCT) obtained by distributing a polymer in cholesteric liquid crystal to stabilize the displaying state. An example of driving cells of this type by a simple-matrix display method to effect memory display will be explained (reference: M. Pfeiffer et al. "A High-Information-Content Reflective Cholesteric Display," SID '95 Digest pp. 706-709).
FIG. 1 shows the reflectivity after a pulse voltage has been applied to PSCT liquid-crystal cells.
In the planar state indicated by P, cholesteric liquid-crystal molecules form domains with the direction of the normal to the twist of each domain being the same. With the planar state, a specific wavelength determined by the pitch of the twists in liquid-crystal molecules is reflected, resulting in a high reflectivity. With the focal conic state represented by f.sub.c, the direction of the normal to the twists in the cholesteric liquid crystal is at random, which causes the light incident on the liquid-crystal panel to scatter, making the reflection intensity lower.
As seen from FIG. 1, both of the planar state and the focal conic state can exist in the area where the applied voltage is low. This means that liquid-crystal molecules can take two stable states without an external electric field and that the liquid crystal itself has memorizing capability, which makes it possible to make a display.
In FIG. 1, the voltages ranging from 0 to V1 are the insensitive voltage region where the initial state remains unchanged before a voltage pulse is applied. The voltages ranging from V1 to V2 are the intermediate voltage region where the planar state (P state) in the initial state changes to the focal conic state (f.sub.c state), with the liquid-crystal molecules presenting the P state and the liquid-crystal molecules presenting the f.sub.c state being distributed at a certain probability according to the cell structure and applied voltage. The voltages ranging from V2 to V3 are the region presenting the f.sub.c state, regardless of the initial state. With the homeotropic state represented by H with the voltages equal to or higher than V4, the helical structure of the liquid crystal has been untied. After being applied with a pulse voltage equal to or higher than V4, the liquid crystal returns to the P state, resulting in a higher reflection intensity. Like the voltages ranging from V1 to V2, the voltages ranging from V3 to V4 are the intermediate voltage region where the P state further changes to the f.sub.c state.
Now, a case where a liquid-crystal panel using the PSCT is driven by a simple-matrix display method will be described by reference to the waveform diagram of FIG. 2 and the circuit diagram of FIG. 3.
In FIG. 2, signal Rv is a signal applied to the liquid-crystal layer by a Y driver 24 of FIG. 3. Signal Sv is a signal applied to the liquid-crystal layer by an XU driver 25a or an XD driver 25b. The liquid-crystal layer 21 at the intersection of a scanning line 22 and a signal line 23 forms a single pixel. The pixel voltage applied to the liquid-crystal layer 21 is Vpix of FIG. 2.
Specifically, signal Rv has the waveform of the scanning signal applied to the direction of row to scan the pixels arranged in a matrix. Signal Sv has the waveform of a display signal applied in the direction of column of pixels to determine whether to bring the pixels to which the scanning signal is being applied into the P state (selected state) or the f.sub.c state (unselected state). The scanning signal is in the "0" state for the pixels not to be scanned.
The phase of the select signal shifts from that of the unselect signal by 180.degree.. To change the display state in the selected state or memory state to the P state, the display signal Sv is applied so that the phase of Rv may be the reverse of that of Sv. To change the display state in the unselected state or memory state to the f.sub.c state, the display signal Sv is applied so that the phase of Rv may be the same as that of Sv.
If the display signal voltage is Vcol and the scanning signal voltage is Vrow, the relationship between the voltage of the scanning signal and that of the display signal has only to fulfill the following: EQU (V4-V3)&lt;Vcol&lt;V1 EQU (2V3-V4)&lt;Vrow&lt;V3
In this case, the voltage of a selected pixel is V4 or higher and the voltage of an unselected pixel is V2 or higher but is less than V3.
Therefore, regardless of whether the initial state is in P or f.sub.c, driving the pixels under the above conditions causes the selected pixel to change its state from the initial state through H to P and goes into the memory state (in the voltage range from 0 to V1). The driving also causes the unselected pixels to change their state from f.sub.c in the voltage-applied state (in the voltage range from V2 to V3) to f.sub.c in the memory state (in the voltage range of 0 to V1) and goes into the memory state. It should be noted that when the initial state is f.sub.c, changing the state to P involves passing through H.
When the simple-matrix display method is used to make a display, the following problems arise.
A first problem is that the time required to rewrite the image on the screen is long. When the PSCT is used, the response time needed for the transition from P to f.sub.c or from H to P is at least several milliseconds of time. Therefore, in the case of 1000 scanning lines, for example, it takes at least several seconds to rewrite the entire screen. The cause of the problem is ascribed largely to driving the simple-matrix liquid-crystal device line by line.
For example, when the initial state is P or f.sub.c and the next state is made P, the initial state is changed to H once and thereafter is changed to P again. Therefore, after the time is allowed to elapse to let a voltage of V4 or higher be applied to change the state to H, it is necessary to secure time to change the state to P again, which requires a lot of time.
A second problem is that the power consumption is great. A voltage of V4 or higher to change the state to H must be applied before the final voltage of "0" to change the state to P is applied. To do this, the signal line has to be charged and discharged at a voltage of Vcol. Namely, the signal line is charged and discharged at the doubled frequency. The doubled frequency doubles the power consumption, resulting in an increase in the power consumption.
A third problem is that a high-quality display is difficult to achieve. With a liquid-crystal display device using cholesteric liquid crystal, the H state in which the spiral structure has been untied has the highest transmittance. Providing an optical absorption layer under the liquid-crystal layer makes the absorbance higher that in the f.sub.c state. In the case of simple-matrix driving, however, the H state cannot be maintained, so that the high absorption state cannot be realized. Consequently, a high-contrast display with P and H cannot be made, making it difficult to provide a high-quality display.